The present invention is related to improved multi-layered ceramic capacitors (MLCC's). More specifically, the present invention is related to multi-layered ceramic capacitors comprising at least two active overlap regions with shields which are not aligned with active electrodes.
In any component comprising two electrodes of opposed polarity there is a tendency towards arcing at given applied voltage. Many factors can affect the voltage at which the arcing occurs with the distance that separates the electrodes being but one key factor. The tendency for surface arcing in air has been exploited to dissipate energy to ground as exemplified in U.S. Pat. No. 5,915,757 wherein described is the relationship between the gap size and arc voltage in air. However, in electronic components, such as capacitors, if arcing occurs it can cause electrical breakdown, which can disrupt, or destroy, other components connected to the capacitor thereby compromising the entire circuit. In most applications surface arcing limits the voltage at which the capacitor can be used.
It is known in the art to apply coatings to either the capacitor, or to the circuit after assembly, to prevent arcing. In addition to the expense associated with additional coatings any mechanical damage to the coating can compromise performance.
Shield electrodes have been applied in lower voltage multilayer ceramic capacitors for ratings less than about 2500V. High voltage rated MLCC 500V) are known to be produced with serial designs and shield designs. Serial designs have 2 or more capacitors in series within the component wherein an even number of capacitors is employed which allows a symmetric electrode print pattern to be employed during manufacture. The voltage is divided between the serial capacitors resulting in high voltage capability at lower capacitance. For any given number of capacitors in series (N) the acting voltage on each capacitor is reduced by the reciprocal of the number of capacitors (1/N) but the effective total capacitance is reduced according to following equation:1/CEff=Σ1/CN.
Serial designs have been used in MLCC's for many years. Typically, an even number of coplanar active electrodes are arranged between floating electrodes so that within a given electrode print plane coplanar active electrodes of opposite polarity contact terminals of opposing polarity and the floating electrodes are parallel to the active electrodes and separated from the active electrodes by a dielectric. The floating electrodes are not in contact with either terminal. These arrangements of electrodes result in an even number of capacitors stacks in series within each MLCC. The serial designs exhibit very high voltage breakdown in inert fluid but in air the breakdown voltage is significantly lower. The inert fluid prevents arcing so the lower voltage breakdowns observed in air are due to surface arcing.
To avoid surface arcing MLCC's rated at greater than 2000V, with X7R dielectric for example, are coated thereby providing an arcing performance in air which is closer to the arcing obtained in inert fluid. However, coatings do have inherent disadvantages. Coatings are expensive either for the MLCC or for the circuit after assembly since it requires additional processing steps and additional materials. Coated MLCC's, or coated circuit boards, are not compatible will all types of subsequent assembly processes which limits the application. Uncoated parts are not easily screened at voltages>2 kV. Mechanical damage to the coating can compromise performance.
High capacitance, high voltage MLCC's, such as 22,000 pF, have been described with a shield electrode and a single electrode overlap area. The internal breakdown voltage is limited to the ability of the active area between the electrodes to sustain high voltage for prolonged periods without failing. Increasing the active thickness does increase the breakdown voltage but at the expense of lowering capacitance. In practice, the shield type designs therefore have an advantage with respect to achieving higher capacitance, such as <2 kV X7R MLCC with X7R dielectric.
In spite of the ongoing efforts those of skill in the art still do not have a suitable option for high capacitance, high voltage, MLCC's which are not susceptible to arcing at lower voltages. Such an MLCC is provided herein.